Introduction:In a generating station the generator and transformer are the most expensive equipments and hence it is desirable to employ a protective system to isolate the faulty equipment as quickly as possible to keep the healthy section in normal operation and to ensure uninterruptable power supply.The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency . Protective relays utilizes one or more of these quantities to detect abnormal conditions in a power system.Protective system cost is 4-5%of the total cost

SWITCHGEARSwitchgear is a general term covering a wide range of equipments concerned with switching and protection.

Eg: Circuit breaker, Isolator, Earth switch etc.

DESIRABLE PROTECTION ATTRIBUTESReliability SelectivitySpeed Simplicity Economics

PROTECTION ZONES *GE Consumer & IndustrialMultilin

Generator or Generator-Transformer UnitsTransformersBusesLines (transmission and distribution)Utilization equipment (motors, static loads, etc.)Capacitor or reactor (when separately protected)

MAIN EQUIPMENT FOR SWITCHGEAR OPERATION

Current transformerPotential transformerRelayCircuit breaker

*GE Consumer & IndustrialMultilin

Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relayVT ratios are typically expressed as primary to secondary; 14400:120, 7200:120A 4160:120 VT has a VTR of 34.66

Voltage Transformers

*GE Consumer & IndustrialMultilin

*GE Consumer & IndustrialMultilin

Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc.CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5A 2000:5 CT has a CTR of 400Current Transformers

*GE Consumer & IndustrialMultilin

AlarmAct at Abnormal condition.Disconnect.Fast operation.Use system supply.

Simple electromechanical relay

Reed relayLatching relaySolid state relaySolid state contact relayRatchet relayCoaxial relayOverload protection relayForced guided contact relayBuchholz relay

It like a fuseIt is a switchInterrupt the faulty partOperation

1.Voltage class2.Current rating3.Type of circuit breaker

1.Air breaker circuit breaker2.Miniature circuit breaker3.Air blast circuit breaker4.SF6 circuit breaker5.Low oil circuit breaker 6.Vaccum circuit breaker

SF6 CIRCUIT BREAKER

PLANT LAYOUT

GENUATAVR220 kv bus220 kv HVCB6.6 KV CBNGT10.5 KV220 KVGTEXT TRSER TR415 V ACLASingle line Diagram of generator connection

GENERATOR THEORY GENERAL OVERVIEW AND TYPICAL SYSTEM

500 MW TG ON TEST BED

NATURE OF FAULTS IN GENERATORInsulation failure.Tends to deteriate with rising temp.Insulation failure may cause inter-turn fault, ph to ph or earth fault.Bring winding in to direct contact with core plates.Any failure to restrict earth fault may result into core plate damage.Insulation of rotor winding is also important.

Fault Occur In Generator Stator FaultRotor faultAbnormal Running ConditionUnbalanced LoadingOver loadingOver SpeedOver VoltageFailure of Primer MoverLoss Of ExcitationExcessive vibrationDifference in expansion between rotating and stationary partsLoss of synchronism

PROTECTION APPLIED TO GENERATORRelays to detect faults outside generatorRelays to detect faults in side generatorOver speed protections.Temp measuring device for bearings, stator winding, Oil temp.

EQUIPMENT GROUNDINGPrevents shock exposure of personnelProvides current carrying capability for the ground-fault currentGrounding includes design and construction of substation ground mat and CT and VT safety grounding

SYSTEM GROUNDINGLimits overvoltagesLimits difference in electric potential through local area conducting objectsSeveral methodsUngroundedReactance Coil Grounded High Z GroundedLow Z GroundedSolidly Grounded

SYSTEM GROUNDING*GE Consumer & IndustrialMultilin

SYSTEM GROUNDING*GE Consumer & IndustrialMultilin

SYSTEM GROUNDING*GE Consumer & IndustrialMultilin

generatorNGTNGRRELAY GENERATOR EARTHING

Stator protection:Stator faults include the following-Phase-to-earth faultsPhase-to-phase faultsInter-turn faultsFrom these phase faults and inter turn faults are lesscommon ,these usually develop into an earth faults.This causes- Arcing to coreDamage of conductor and insulation

INTER-TURN FAULT PROTECTION

Stator inter-turn fault protection:Inter-turn fault on the same phase of the stator winding cannot be detected by transverse differential protection as it does not disturb the balance between the currents in neutral and high voltage CTs.For protection against inter-turn faults the following protection schemes are used. (1)Cross differential protection. (2)Residual voltage protection.

mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmLoading resistorOver voltage relayWith time delaySTATOR EARTHFAULT RELAY

exciterP.BField wdgVoltage relayROTOR E/F RELAY

Rotor earth fault protection:DC injection method or AC injection method.The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac).But dc source is generally used as over-current relay in case of dc is more sensitive than ac.A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay.

Rotor earth fault protectionAC Injection method

GENERATOR PROTECTION 1 ST ROTOR E/F PROTECTION (64R1)D.C. INJECTION METHOD.

Rotor temperature alarmIt is provided in large generators.It indicates the level of temperature but not the actual hot spot temperature.The relay measures the temperature by measuring the resistance .(as shown in fig)

GENERATOR PROTECTIONAbnormal Operating Conditions

Loss of excitation protection:When the excitation of generator is lost it operate as aInduction generator. It derives excitation from thesystem and supply power at leading power factor. Which may cause- A fall in voltage & so loss of synchronism & system instability.Over heating of rotor due to induction current on it.A protection having MHO characteristicis used to detect loss of field.

Differential protection of generator:

Differential protection using balancing resistor:

Modified differential protection

Modified differential protection:Generally protection is made for 80 to 85% of the winding.If any fault occurs near the neutral point then the fault current is very small and relay does not operate.Modified differential protection scheme is used to over come this.Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.

External fault back-up protection

External fault back up protection:Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection.Induction type IDMT relay is used for this purpose.

STEAM VALVEC.B TRIPProtective relayReverse power relayReverse power relay scheme

REVERSE POWER PROTECTIONFailure of the prime mover of a generator set ,will keep the set running as a synchronous compensator, taking the necessary active power from the net work and could be detrimental to to the safety of the set, if maintained for any length of time. The amount of power taken will depend on the type of prime mover involved. It ranges from 5% to 25%.

mm46mmZcZAABCIaIbIcVZCVZAPOSITIVE SEQIaIbIcVZCVZAVZA+VZCXYNEGATIVE SEQUENCENegative phase sequence protection

Negative phase sequence protection:Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current .This current in rotor causes rotor overheating and damage to the rotor.This can be protected by negative sequence current filter with over current relay.

Negative phase sequence protection:

ExciterFUSET1T2FUSETRIPSHUNTFILED WDGField failure protection

FIELD FAILURE PROTECTIONLoss of generator field excitation under normal running conditions may arise due to any of the following condition.1. Failure of brush gear.2. unintentional opening of the field circuit breaker.3. Failure of AVR. When generator on load loses its excitation , it starts to operate as an induction generator, running above synchronous speed. cylindrical rotor generators are not suited to such operation , because they do not have damper windings able to carry the induced currents, consequently this type of rotor will overheat rather quickly.

Over voltage protection:Overvoltage protection is required in case of hydro-electric or gas turbine generators but not in case of turbo generators.Over voltage may be caused due to-Transient over voltage in the transmission line due to lightening.Defective operation of the voltage regulator.Sudden loss of load due to line tripping.The protection is provided with an over voltage relay.It is usually of induction pattern with an IDMTCharacteristic

Overcurrent protection:Overloading of the machine causes overheating in the stator winding.This can be prevented by using over-current relay with time delay adjustment.But overheating not only depends on over-current but also the failure of the cooling system in the generator.So temperature detector coils such as thermistors or thermocouples are used at various points in stator winding for indication of the temperature.

GENERATOR PROTECTION

Name Input Protecting toDifferential protectionDifferential CurrentStator core and windingStator earth faultVoltageStator core and windingOver currentCurrentStator core and windingOver voltageVoltageStator core and windingInterturn short circuit CurrentStator core and windingRotor Earth FaultCurrentRotor windingOver and under frequencyFrequencyTurbine protectionReverse power flowVoltage and currentTurbine protection

Loss of excitationVoltage and currentPower System ProtectionBack up protection for linesVoltage and currentGenerator protection

****Current Turns Ratio (CTR)SR489*******SR489**